Uranium-235 has to be enriched before it can be turned into fuel rods. this enrichment is achieved by *freezing it. by *converting into gas.

Uranium 235 has to be enriched before it can be turned into fuel rods. This enrichment is achieved by converting it into gas and freezing it.  

Explanation:

Uranium 235 is the isotope used in nuclear reactors. Naturally obtained Uranium cannot be as such used in a nuclear reactor. Mined Uranium consists of Uranium 238 in the major proportion. Uranium 235 is found in lesser amount.  

Initially Uranium oxide is converted to Uranium hexafluoride which is a gas at suitable operating temperatures. It is this UF which is used in the enrichment facility. Gaseous diffusion, gas centrifuge and laser diffusion are some of the enrichment processes.  

In gaseous diffusion method lighter UF containing U and U diffuses faster after passing through several barriers and the collected UF is condensed. It is then poured into containers where it is solidified.

definitely a for sure. ill see and update if i get the answer